1. Field of the Invention
The present invention relates to a printing apparatus that prints an image on an object, such as a print medium, while conveying the object relative to a print head and also to a method of controlling a conveyance of an object such as a print medium. Particularly the present invention relates to a construction and a method for detecting a distance that the object, such as print medium, has traveled and a speed of the object with high precision.
2. Description of the Related Art
In a printing apparatus that prints an image on a print medium using a print head while moving a print medium, to print a high-quality image with higher precision calls for raising a print medium conveyance precision. For example, U.S. Pat. Nos. 5,149,980 and 7,104,710 disclose a technology to optically measure the actual distance traveled by the print medium.
FIG. 1 shows an apparatus, disclosed in U.S. Pat. No. 7,104,710. The apparatus disclosed in U.S. Pat. No. 7,104,710 has two measuring means a means to measure the real rotational amount of a conveying roller that conveys the print medium and a means to measure the real distance traveled by the print medium.
In FIG. 1, a print medium 107 is held between conveying rollers 101, 102 and opposing two pinch rollers 103, 104 and conveyed by the rotation of the conveying rollers 101, 102 in a Y direction. A rotating force of the conveying rollers 101, 102 is produced by a drive shaft of a conveying motor 108 engaging the two conveying rollers. A code wheel 105 is secured on the same rotary axis as the conveying roller 101.
A rotation angle sensor 106, installed at a position where a circumferential portion of the code wheel 105 passes, can measure the rotational amount of the code wheel 105. From the rotational amount of the code wheel 105, i.e., the rotational amount of the conveying roller 101, the distance traveled by the print medium 107 can be determined.
It is noted, however, that the rotation angle of the conveying roller does not necessarily match the actual distance that the print medium was conveyed. An eccentricity of the conveying roller that may occur during its installation and a slip between the conveying roller and the print medium unavoidably cause some shifts or deviations between the distance traveled by the print medium and the rotation angle of the conveying roller. To avoid this problem, U.S. Pat. No. 7,104,710 discloses a construction which has, in addition to the rotation angle sensor 106, an optical sensor 701 that measures the actual distance traveled by the print medium 107 and performs the print medium conveyance control based on conveyance information from the two sensors.
In the example shown, the optical sensor 701 is mounted on a carriage 200 along with a head cartridge 100 and disposed between the two conveying rollers 101, 102. The optical sensor 701 captures states of a surface of the print medium being conveyed as image information at a plurality of timings. From the plurality of pieces of image information thus captured, a control unit of the printing apparatus calculates the distance traveled by the print medium and the conveying speed of the print medium. As described above, the provision of a means to directly detect the conveying distance and speed of the print medium and the driving of the conveying means according to the information obtained allow the printing position of an image on the print medium to be controlled more precisely.
It is noted, however, that even if a means to directly detect the conveying distance of a print medium is provided, as in U.S. Pat. Nos. 5,149,980 and 7,104,710, the conveying speed cannot be detected at timings at which the print medium is outside a detectable range of the optical sensor, such as at an initial or final stage of the conveying operation.
Take for example a case where the optical sensor 701 is used, as shown in FIG. 1. At an initial stage of the conveying operation until the print medium 107 comes under the optical sensor 701, the distance traveled by the print medium 107 cannot be measured. As a result, when a front part of the print medium is printed, the information from the optical sensor 701 cannot be reflected on the driving of the conveying motor 108, giving rise to a possibility that the printing position of the front part of the print medium may not be controlled precisely.